The present invention relates to a contact structure for a vacuum circuit breaker used for temporarily interrupting power supplying when an abnormality occurs in power substations, factories and other buildings.
As shown in FIG. 1, a conventional contact structure for a vacuum circuit breaker comprises two electrically insulating cylindrical members 1a-1, 1a-2 which are axially disposed and end plates 2 and 3, provided at outer ends of the cylindrical members 1a-1 and 1a-2, respectively, for forming an evacuated vacuum vessel 1. A fixed contact 4 is connected with a first end portion of an electrically conductive rod 4a having a second end portion which passes through the end plate 2 in an airtight manner. A movable contact 5 is connected with a first end portion of an electrically conductive rod 5a having a second end portion which is movably and hermetically sealed to the end plate 3 through bellows 6. The fixed contact 4 and the conductive rod 4a form a fixed contact unit while the movable contact 5 and the conductive rod 5a form a movable contact unit.
A first end shield member 7-1 is provided for the fixed contact 4. A central shield member 8 is provided in the center of the vacuum vessel 1. A second end shield member 7-2 is provided for the movable contact 5. In other words, the central shield member 8 surrounds the first end portions of the conductive rods 4a and 5a and the contacts 4 and 5. The first end shield member 7-1 surrounds the second end portion of the conductive rod 4a. The second end shield member 7-2 surrounds the second end portion of the conductive rod 5a. These shield members 7-1 and 7-2 play an important role to prevent the vapor of a metal generated between the contacts 4 and 5 on current interruption from being deposited upon the inner wall of the vacuum vessel 1. However, the breakdown voltage is lowered since the electrically insulating cylindrical members 1a-1, 1a-2 are disposed in the vicinity of the end shield members 7-1, 7-2 and the central shield member 8. This is due to a fact that a secondary electron avalanche takes place on the inner surfaces of the insulating cylindrical embers 1a-1 and 1a-2 when a high voltage is applied to the contact structure. A countermeasure against this has been proposed that wherein the insulating cylindrical members 1a-1 and 1a-2 are coated on the inner side thereof with a material having a secondary electron emission efficiency .delta. not greater than 1 such as chromium oxide (Cr.sub.2 O.sub.3). For example, refer to JP-A-60-93721 (laid-open May 25, 1985). A method has been proposed in which the insulating cylindrical members 1a-1 and 1a-2 are coated with films of chromium oxide 1b-1 and 1b-2, respectively only on the inner side thereof facing a gap between the central shield member 8 and the end shield members 7-1 and 7-2. The coating film is 0.1 .mu.m to 5 .mu.m in thickness and the coating is performed by vacuum deposition or sputtering. This enables the electric charges on the inner side of the insulating cylindrical members 1a-1 and 1a-2 to be neutralized for increasing the breakdown voltage and the period of time required for the coating process to be shortened and can provide a contact structure for a vacuum circuit breaker which is economical and has a high blocking voltage and a high capacity.
The resistivity of chromium oxide at room temperatures is in the order of 10.sup.3 .OMEGA.cm. If the insulating cylindrical member is coated with chromium oxide on the inner side facing the gap between the central shield member 8 and the end shield members 7-1 and 7-2 to form a coating layer having a thickness of 0.1 .mu.m to 5 .mu.m, the electric resistance of the coating layer is generally not higher than 10M.OMEGA.. The electrons emitted from the central shield member 8 and the end shield members 7-1 and 7-2, in particular from the end thereof will impinge upon the surface of the chromium oxide layers 1b-1 and 1b-2 to elevate the temperature on the surface thereof. Chromium oxide has negative temperature characteristics in which the resistivity is lowered with an increase of the temperature. The surface temperature of the layers 1b-1 and 1b-2 will be elevated due to collision with electrons when the contact structure is in operation with a voltage applied thereto, and the electric resistance of the chromium layer will be further lowered. If an overvoltage is applied to the contact structure of the vacuum circuit breaker which is kept in operation with a voltage applied thereto for a long period of time, no flashover occurs at an initial stage, but a flashover may occur between the central shield member 8 and the end shield members 7-1 and 7-2 via the chromium oxide layers 1b-1 and 1b-2.
Meanwhile, if coating with chromium oxide is achieved by vacuum deposition or sputtering, a large amount of impurities will be generated inside of an evaporation apparatus or a sputtering apparatus, so that the life time of the apparatus may be shortened. The melting point of chromium oxide is as high as 2320.degree. C. A relatively long period of time is required to perform a coating by vacuum deposition or sputtering. Coating with chromium oxide thus has various problems.